Israeli gunboats shelled a Syrian controlled area of western Beirut last night in Israel's first open intervention in the escalating war between Syrian troops and Lebanese Christian militias.

The Israeli military command announced that its gunboats fired on a Palestinian naval base near a beach club that has been turned into a Palestinian refugee shelter north of Beirut International Airport. Washington Post correspondent William Claiborne reported from Tel Aviv.

Correspondents in Beirut, however, pinpointed the attack about a quarter of a mile farther north near a beachside avenue lined with multistory apartment buildings, and said Syrian artillery returned the fire.

Both areas have been under the control of Syria's peacekeeping troops since November 1976. The Israeli shelling which followed repeated expressions of Israeli concern for the beleaguered Lebanese Christians brought Tel Aviv a step closer to its first direct confrontation with Syria since the 1973 Middle East war.

The Israeli shelling seemed to be a clear warning to Syrian President Hafez Assad who has resisted a combined effort by the United States, France and Saudi Arabia to arrange a cease-fire. Israel has backed the Christian irregulars with arms training and political support aimed at preventing Lebanon from becoming a potential pro-Syrian front in the event of another Middle East war.

A high U.S. official with Secretary of State Cyrus Vance at the United Nations said President Carter sent a letter to Assad yesterday stressing U.S. concern over the Beirut fighting and Carter's strong support for an immediate cease fire.

The official said that if the cease-fire is not agreed by today, the United States "would probably" ask for an [WORD ILLEGIBLE] Security Council meeting to seek a resolution for the cease-fire. The Carter administration has been in touch with the Soviet Union on the possible council meeting he said but declined to assess the liklihood of Soviet cooperation.

The Israeli announcement made no mention of the fierce shelling between the Syrians and Lebanese Christian militias which continued through another day of heavy civilian casualties and destruction of residential buildings, news agency dispatches said.

There were no Israeli casualties," said the terse Israeli communique which officials in Tel Aviv refused to elaborate on.

There were no reports on casualties from Beirut. But correspondents there reported that ambulances sped to the predominantly Moslem Ramlet al-Baida area where three Israeli gunboats attacked.

The attack was Israel's first in Lebanon since Israeli warplanes bombed and [WORD ILLEGIBLE] Palestinian guerrilla encampments near the coast of central Lebanon Aug 3 following a terrorist bomb explosion in a crowded Tel Aviv marketplace.

But that raid, like Israel's invasion of southern Lebanon last March did not occur in areas occupied by Syrian troops and was not interpreted as Israeli intervention against Syria in the Lebanese conflict.

In Jerusalem Defense Minister Ezer Weizman briefed the Israeli Cabinet. He declined to make any comment afterward and the content of his briefing was classified secret.

The Israeli army radio station quoted sources in the Israeli parliament as saying that "the decisive moment is near" for Israel, the AP reported. The radio also said Weizman would report today to an emergency meeting of the parliament's defense and foreign affairs committees.

Israel has been issuing warnings on the Lebanese situation since July, Claiborne reported. That was when the level of fighting escalated between the Maronite Christian militias and the 30,000 Syrian troops in Lebanon as the backbone of an Arab League peacekeeping force.

Israeli jets flew over Beirut breaking the sound barrier at rooftop levels several times. As they did officials in Jerusalem were saying that Israel would not "stand idly by" while Lebanese Christians were killed.

However, in the weeks preceeding the Camp David summit conference, the Israeli government was mute about the Syrian attacks even though the intensity of the fighting in Beirut increased. Observers had noted Israel's silence as the Beirut fighting escalated sharply this weekend.

The first mention of possible Israeli intervention on behalf of the Christians since the beginning of the summit occurred yesterday when Deputy Prime Minister Yigael Yadin said. "The situation in Lebanon is grave all the time. Whatever will be needed, whatever will have to be done will be done."

Israeli Christians have been appealing for international help. Lebanon former president Charles Malik, a top Maronite leader, said in Washington yesterday that he supported the French-U.S. cease fine efforts.

The cost of the fighting has been high to Christian civilians but casualties have been limited to about 50 among the young Lebanese Christian street fighters, according to Dory Chamoun, a leader of the Christian National Liberal Party.

Christian sources in Beirut told Washington Post special correspondent William Branigin that about 700 people, nearly all civilians, have been killed since Saturday.

Predominantly Christian East Beirut and its southern suburbs are in ruins with an estimated 20,000 houses and apartments destroyed and the mounds of rubble growing every day.

There is no more running water or electricity. Even in parts of the predominantly Moslem western half of the capital, food, gasoline and other items are running short and a sooty cloud hangs over the city from an oil storage tank fire that has been burning since Tuesday.

"Nobody cares what's happening to us," cried an anguished Christian woman by telephone from a relative's home on Mount Lebanon. Like thousands of others, she and her family had fled their apartment in East Beirut during previous shelling only to find themselves under fire again when the fighting spread at the weekend to the hills north and east of the capital.

"Why doesn't the United Nations or America do something to end this hell," said another resident from an East Beirut shelter.

So far, however, appeals by the United States for a peace conference on Lebanon and by France for a ceasefire and the establishment of a Lebanese army buffer force between the militias and the Syrians, have met with no success.

The hard-line militia leaders have welcomed "internationalization" of the conflict, provided it leads to Syrian withdrawal from Lebanon. The Syrians have effectively rejected both the U.S. and French proposals, the Lebanese government has maintained a noncommittal posture.

The only public reaction to the French appeal by Syrian President Hafez Assad - before he flew to Moscow Thursday to coordinate Soviet and hard-line Arab opposition to the Camp David accords - was to call it "strange." He insisted that his 30,000 plus soldiers in Lebanon are commaned by the Lebanese government as its "legal" security force.

This is true enough on paper, but in practice the government of President Elias Sarkis has absolutely no control over the Syrian troops, who take their orders from Damascus.

Sarkis however, has never publicly said so. Though he is known to be deeply distressed by the bombardments, he has officially neither repudiated the Syrian troops nor appealed for outside help to stop them. In fact he has called for the renewal of the Arab peacekeeping force's mandate, which expires Oct. 26.

Right-wing Christian leader Camille Chamoun, who insists the mandate not be renewed had described Sarkis as a "prisoner" of the Syrians, who backed his presidency. But senior Western diplomats say Sarkisi dilemma stems from the likelihood - if not inevitability - of renewed civil war should the Syrians depart.

Under these circumstances, diplomats say there is little that Western countreis can do to curb what they call a "crude" and "heavy-handed" Syrian reaction to militia provocations. According to these sources, neither the French nor the Americans are in a position, even if they wanted to send troops in as the United States did at the government's request in 1958 to halt civil strife. In addition, they say, there is virtually no chance of U.N. intervention, if only because a U.N. peacekeeping force for Beirut would be vetoed by the Soviet Union, which supports Syria.

The Syrians already have rejected the idea of pulling back from East Beirut and other Christian strong holds in favor of a joint Moslem-Christian Lebanese army force. Not only would that leave the Christian rightists political domination of Lebanon unchecked, but it would be a serious loss of face for the Syrian army.

There also are growing doubts about the ability of the post-civil war Lebanese army to remain intact under sectarian pressures.Although Western military sources deny rightist allegations that entire units have defected to join the battle against the Syrians, they say a number of individual soldiers have done so and the threat of a major split is developing.

"Our worst fears have not yet been realized," one official said "But if the shelling" goes on much longer it's doubtful the army will hold together."

A diplomat said that while the Syrian army's prestige might suffer from a pullback, "razing half a city just to put down a bunch of militiamen isn't too good for its image either."